Support private values in GN.

Support private values in GN.

This adds special handling for variables that begin with underscores, which is inspired by Dart. Such variables are not imported when doing an import, which gives .gni files a way to have intermediate private variables that won't pollute the scopes of the files that include them.

This also applies to the root build config, which can have private values now.

Adds some missing unused variable checks. This was disabled because processing imports would mean all imported variables were unused, and files not using all of them would get unused variable errors. This adds the option to mark merged values on a scope as used, which is used for imported values.

BUG=341738
R=cjhopman@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=271078

[cjhopman, 2014-05-14 16:42:43]

This is nice, but it limits "modules" to be single files. 

I basically want foo.gni to be able to import foo_internal.gni (and maybe other
foo implementation details files) and keep private variables (which will be
cleared for importers of foo.gni).

Dart supports this with library/part declarations but we could probably do
something simpler than that (and I don't think this change makes it particular
hard to come up with something that works).

https://codereview.chromium.org/287693002/diff/20001/tools/gn/import_manager.cc
File tools/gn/import_manager.cc (right):

https://codereview.chromium.org/287693002/diff/20001/tools/gn/import_manager....
tools/gn/import_manager.cc:91: options.mark_used = true;  // Don't require all
imported values be used.
There's no tests that mark_used=true works.

https://codereview.chromium.org/287693002/diff/20001/tools/gn/loader.cc
File tools/gn/loader.cc (right):

https://codereview.chromium.org/287693002/diff/20001/tools/gn/loader.cc#newco...
tools/gn/loader.cc:24: void ClearPrivateVariables(Scope* scope) {
Why not just add this on Scope?

https://codereview.chromium.org/287693002/diff/20001/tools/gn/scope.h
File tools/gn/scope.h (right):

https://codereview.chromium.org/287693002/diff/20001/tools/gn/scope.h#newcode73
tools/gn/scope.h:73: copy_private_vars(true),
nit: how about clear_private_vars so that the defaults can all be false?

[brettw, 2014-05-15 19:44:15]

New snap up.

https://codereview.chromium.org/287693002/diff/20001/tools/gn/import_manager.cc
File tools/gn/import_manager.cc (right):

https://codereview.chromium.org/287693002/diff/20001/tools/gn/import_manager....
tools/gn/import_manager.cc:91: options.mark_used = true;  // Don't require all
imported values be used.
On 2014/05/14 16:42:43, cjhopman wrote:
> There's no tests that mark_used=true works.

I added one, thanks.

https://codereview.chromium.org/287693002/diff/20001/tools/gn/loader.cc
File tools/gn/loader.cc (right):

https://codereview.chromium.org/287693002/diff/20001/tools/gn/loader.cc#newco...
tools/gn/loader.cc:24: void ClearPrivateVariables(Scope* scope) {
On 2014/05/14 16:42:43, cjhopman wrote:
> Why not just add this on Scope?

Done.

https://codereview.chromium.org/287693002/diff/20001/tools/gn/scope.h
File tools/gn/scope.h (right):

https://codereview.chromium.org/287693002/diff/20001/tools/gn/scope.h#newcode73
tools/gn/scope.h:73: copy_private_vars(true),
On 2014/05/14 16:42:43, cjhopman wrote:
> nit: how about clear_private_vars so that the defaults can all be false?

Done.

[cjhopman, 2014-05-15 21:16:49]

lgtm

[brettw, 2014-05-16 19:31:41]

Committed patchset #4 manually as r271078 (presubmit successful).